This invention relates to a device incorporated in the wheels of cross-country vehicles and machines, for example for forestry, which is capable of increasing the grip necessary for advancing the vehicle or machine when increased traction is required.
Due to the ever increasing mechanization of forestry in recent times, not only the number of machines used in forestry and agriculture has increased, but in many cases also the size and weight of the machines have increased, in spite of the fact that so far no solution has been found of how to cope with the problem of damages caused on soil and growing trees, and particularly on the root system of the latter, by said machines, especially when they are advanced on bare ground. Forestry vehicles at present are driven either by means of large wheels equipped with profiled tires and suspended in a bogie structure or by means of steel tracks in combination with such profiled tires. Compared with profiled tires alone, which produce a high local surface pressure and have poor bearing capacity, so that the machine easily will sink down into the soil and form remaining furrows, especially when the soil is relatively weak, the use of steel tracks in combination with profiled tires has resulted in an improved grip and a wider spread of load and force, whereby the feasibility of using such vehicles on weak soils is enhanced. Steel tracks, however, have proved relatively aggressive and by their structure to expose the soil to heavy point loads, which give rise to damages of growing trees, particularly their root systems, and leaves furrows in the soil, especially when the vehicle is skidding, i.e. when the grip is not sufficient for the forces required for advancing the vehicle. A further disadvantage of steel tracks is that they must be removed every time the vehicle is to be driven on a paved road. Steel tracks, moreover, when they come into contact with stones and the like give rise to sparks, which can cause forest fires.
It is known that the aforesaid damages to a great extent can be reduced substantially when instead of tracks of steel, tracks of rubber, with or without steel reinforcements, are used. Experiments made with such rubber tracks, however, have not shown satisfactory results. It was found among other problems that solid-rubber tracks, due to the great forces required for advancing forestry machines, stretch too much and thereby give rise to sliding between track and drive wheel. Even rubber tracks with built-in steel reinforcements proved unsatisfactory from a strength point of view, especially when the vehicles are driven up-grade or on soil with difficult surface structure where great traction forces are required.
The present invention has the object, among others, of rendering possible the use of rubber tracks, with or without built-in steel reinforcements, even for cross-country vehicles and machines, for example in forestry, and to produce a device incorporated in the wheels of such vehicles and machines, which is so constructed that, if required, the grip can be increased, more precisely so that the increase in traction forces caused by such increase in grip does not affect a track provided about the wheel.